1. Field of the Invention
The present invention relates to a microwave or millimeter-wave integrated circuit using a semiconductor substrate, commonly known as a microwave monolithic integrated circuit, and more specifically to an oscillation circuit for generating microwave or millimeter-wave signals with circuit operation stabilized according to load conditions.
2. Description of Related Art
Oscillators used in sensors and communications devices operating in the microwave and millimeter-wave bands, referred to below simply as high frequency bands, require a low phase noise characteristic. Phase noise in an oscillator is a variation in the oscillation frequency with a period longer than that of the oscillation frequency; low phase noise is indicative of stable oscillator operation. Factors contributing to phase noise include bias, thermal noise, and modulation caused by nonlinear operation of active elements.
Changes in load impedance can also cause the oscillation frequency of an oscillator to vary, a phenomenon known as pulling. To avoid problems caused by pulling, variations in load are removed by, for example, connecting the oscillator to an amplifier connected to multiple stages or to an isolator.
However, because of the difficulties in manufacturing a resonance circuit with a Q sufficiently high to assure stable oscillator operation in such high frequency band regions as the millimeter-wave band, oscillators operating in such high frequency bands are subject to even greater loads.
It is to be noted that use of Lange couplers is taught in Japanese Patent Laid-Open Publication Nos. 10-242719 and 6-177648, and Japanese Patent Laid-Open Publication 9-46225 teaches a common phase-locked oscillator (PLO).